1. Field of the Invention
This invention relates to an image forming apparatus (electrophotographic apparatus) such as a copying machine, a printer, a facsimile machine or a platemaking system, which employs an electrophotographic process.
2. Related Background Art
In recent years, various approaches are taken because of an increasing need for the achievement of ultrahigh image quality in regard to images reproduced from the image forming apparatus. In particular, the exposure process that forms an electrostatic latent image on the surface of an electrophotographic photosensitive member is positioned on the upstream side in the electrophotographic process, and is the basis of image formation. Accordingly, this process is considered to be an especially important process in order to achieve high image quality of electrophotographic images. Then, making beam spot diameter small in the exposure process enables achievement of ultrahigh resolution, and is a very effective means for the achievement of ultrahigh image quality.
Near infrared region semiconductor lasers having conventionally been used have lasing wavelengths of about 650 to 780 nm, and have spot diameter of about 100 μm. Its limit has been about 50 to 80 μm whatever improvements are made on various optical members in order to make the beam spot diameter small. Also, even if improvements on various optical members have made the beam spot diameter small, it is difficult to obtain the sharpness of a contour of the beam spot. This is known from the diffraction limit of laser beams that is represented by the following equation (48). The following equation (48) shows that the lower limit of beam spot diameter (D) of a beam spot is proportional to the wavelength (λ) of the laser beam. (NA is the numerical aperture of a lens.)D=1.22λ/NA  (48)
Accordingly, it is contemplated to use as an exposure light source (a writing light source) of the electrophotographic apparatus a short-wavelength blue (purple) semiconductor laser, which is being put into practical use in DVD and so forth in recent years (see, e.g., Japanese Patent Application Laid-open No. H9-240051, page 2, claim 1). Compared with the conventional near infrared region semiconductor lasers, in the case when the blue (purple) semiconductor laser having about a half lasing wavelength (380 to 500 nm) is used as an exposure light source, the beam spot can be made to have a very small spot diameter in the state the sharpness of the contour of the beam spot is maintained, as shown in the above equation (48). Hence, this enables achievement of ultrahigh resolution, and is very effective for the achievement of ultrahigh image quality.
Thus, the use of the blue (purple) semiconductor laser as an exposure light source makes it possible for the surface of an electrophotographic photosensitive member to be irradiated with a laser beam in a spot diameter of about 40 μm or less in the state the sharpness of its contour is maintained.
Accordingly, in an electrophotographic apparatus having such a blue (purple) semiconductor laser as an exposure light source and made to have a small beam spot diameter, an electrophotographic photosensitive member having a certain or higher sensitivity to light irradiation of an image exposure device is required as a matter of course. Further, in order for the electrophotographic photosensitive member to effectively utilize the light with which it is irradiated, the photosensitive member is required to have a high spectral sensitivity in the wavelength region of the light source.
However, very few electrophotographic photosensitive members have such a high spectral sensitivity in the wavelength region of the light source. For example, Japanese Patent Application Laid-open No. H10-239956, page 5, discloses a report concerning a selenium (Se—Te) photosensitive member which is an inorganic photosensitive member having a maximum spectral sensitivity at a wavelength of about 460 nm.
Meanwhile, in these days, various studies are made which take note of organic photosensitive members having various advantages that they have a good environmental adaptability, can be manufactured and handled with ease, and enjoy a low cost.
For example, Japanese Patent Application Laid-open No. H8-87124 (see page 2, claim 1) discloses an embodiment of an azo pigment making use of a coupler having a similar structure as the present invention. Japanese Patent Applications Laid-open No. H4-147265 (page 8), No. H2-118581 (page 14) and No. H4-81858 (page 13) disclose embodiments of an azo pigment making use of a central skeleton having a similar structure as the present invention. In these cases, however, what is targeted is white light of a halogen lamp or the like as an exposure means, and there is no disclosure at all that it is applied to the use targeted on the blue (purple) semiconductor laser.
In regard to an azo pigment targeted on the blue (purple) semiconductor laser, Japanese Patent Application Laid-open No. H10-239956 (page 3 and FIG. 4 on page 6) discloses an embodiment making use of an anthraquinone type azo pigment, Japanese Patent Application Laid-open No. 2002-14482 (page 2, claims 1 to 4) and Japanese Patent Application Laid-open No. 2002-131951 (pages 2 and 3, claim 1) disclose embodiments making use of azo pigments having various central skeletons, and Japanese Patent Application Laid-open No. 2000-105478 discloses embodiments making use of azo pigments having various couplers. In these cases, however, those having sufficient sensitivity for the blue (purple) semiconductor laser are not seen in the azo pigments having the combination disclosed in the publications.
Accordingly, it follows that images are reproduced in the state the amount of laser light is made extremely large in order to secure the necessary sensitivity. In such a case, the running potential may vary so greatly as to be insufficient for the reproduction of stable images with ultrahigh image quality throughout running. At the same time, there also are various disadvantages that the reliability of lasers to reproduction stability may lower, a high laser cost may result and the laser may have a short lifetime. Moreover, there is a limit to laser power, and proper sensitivity can not always be secured.
On account of the foregoing, it has been sought to use an organic photosensitive member having a high spectral sensitivity for the semiconductor laser light source having the wavelengths of 380 to 500 nm.